2.4 Blade Enclosure

The enclosure (or chassis) performs many of the non-core computing services found in most computers. Non-blade computers require components that are bulky, hot and space-inefficient, and duplicated across many computers that may or may not be performing at capacity. By locating these services in one place and sharing them between the blade computers, the overall utilization is more efficient. The specifics of which services are provided and how vary by vendor.

Power

Computers operate over a range of DC voltages, yet power is delivered from utilities as AC, and at higher voltages than required within the computer. Converting this current requires power supply units (or PSUs). To ensure that the failure of one power source does not affect the operation of the computer, even entry-level servers have redundant power supplies, again adding to the bulk and heat output of the design.

The blade enclosure's power supply provides a single power source for all blades within the enclosure. This single power source may be in the form of a power supply in the enclosure or a dedicated separate PSU supplying DC to multiple enclosures [1]. This setup not only reduces the number of PSUs required to provide a resilient power supply, but it also improves efficiency because it reduces the number of idle PSUs. In the event of a PSU failure the blade chassis throttles down individual blade server performance until it matches the available power. This is carried out in steps of 12.5% per CPU until power balance is achieved.

Cooling

During operation, electrical and mechanical components produce heat, which must be displaced to ensure the proper functioning of the components. In blade enclosures, as in most computing systems, heat is removed with fans.

A frequently underestimated problem when designing high-performance computer systems is the conflict between the amount of heat a system generates and the ability of its fans to remove the heat. The blade's shared power and cooling means that it does not generate as much heat as traditional servers. Newer blade enclosure designs feature high speed, adjustable fans and control logic that tune the cooling to the systems requirements.[2]

At the same time, the increased density of blade server configurations can still result in higher overall demands for cooling when a rack is populated at over 50%. This is especially true with early generation blades. In absolute terms, a fully populated rack of blade servers is likely to require more cooling capacity than a fully populated rack of standard 1U servers.

Networking

Computers are increasingly being produced with high-speed, integrated network interfaces, and most are expandable to allow for the addition of connections that are faster, more resilient and run over different media (copper and fiber). These may require extra engineering effort in the design and manufacture of the blade, consume space in both the installation and capacity for installation (empty expansion slots) and hence more complexity. High-speed network topologies require expensive, high-speed integrated circuits and media, while most computers do not utilise all the bandwidth available.

The blade enclosure provides one or more network buses to which the blade will connect, and either presents these ports individually in a single location (versus one in each computer chassis), or aggregates them into fewer ports, reducing the cost of connecting the individual devices. These may be presented in the chassis itself, or in networking blades[3].

Storage

While computers typically need hard-disks to store the operating system, application and data for the computer, these are not necessarily required locally. Many storage connection methods (e.g. FireWire, SATA, SCSI, DAS, Fibre Channel and iSCSI) are readily moved outside the server, though not all are used in enterprise-level installations. Implementing these connection interfaces within the computer presents similar challenges to the networking interfaces (indeed iSCSI runs over the network interface), and similarly these can be removed from the blade and presented individually or aggregated either on the chassis or through other blades.

The ability to boot the blade from a storage area network (SAN) allows for an entirely disk-free blade. This may have higher processor density or better reliability than systems having individual disks on each blade.